In some known communication systems, the portable device is of the active kind, in which an action of the user on the portable device, such as pressing a button on the portable device, is required to actuate the transmission of a signal over an electromagnetic transmission path to the base station. The portable device may therefore be totally de-energised until the user actuates transmission of the signal, which avoids unwanted consumption of energy except when the user actuates the device. However, actuation of the device in this way by the user is additional to the actions associated with his primary purpose (such as opening a door, starting an engine or starting use of the data processing equipment for example) and accordingly is inconvenient.
The present invention relates to a communication system of the so-called ‘passive’ kind comprising a communication device capable of transmitting a coded identification signal to the base station without specific action of the user on the communication device in response to reception of an interrogation signal from the base station. As used herein, the expression ‘passive communication device’ includes a device provided with means that may be actuated by a user to trigger transmission to the base station without reception of an interrogation signal from the base station, in addition to its normal passive operation.
A communication system in accordance with the present invention is particularly, but not exclusively, applicable to an access control system and, more particularly, to an access control system of the kind including a portable device which identifies the user of the device to the base station by transmitting a coded identification signal to the base station. The base station may include a mechanism coupled to a door or other closure in order to unlock the door and enable physical entry of the user in response to a valid identification signal received from the portable device; the base station may perform other types of access control, such as control of the enabling of an operational function, and in particular starting an engine or access to use of data processing equipment, for example.
Since the communication device in a passive system must be ready to receive the interrogation signal from the base station and so is energised normally at all times, this can require significant power consumption which is a problem particularly for battery operated devices.
An access control system including a passive communication system is described in the article reference 980381 by Stephan Schmitz and Christopher Roser entitled “A New State-of-the-Art Keyless Entry System, published by the Society of Automotive Engineers in 1998. This publication recognises the need to reduce the power consumption of the transponder described to very low levels but fails to indicate how this objective can be achieved by the internal construction of the transponder.
The present invention is also applicable to other communication systems in which a passive battery powered communication device is responsive to an interrogation signal from a base station, for example, in a system for monitoring the pressure of the tyres of a vehicle: the base station is installed on the bodywork of the vehicle while the communication device is installed in the tyre, coupled with a pressure sensor.
There is therefore a need to obtain a low level of power consumption of the communication device, especially in a standby mode and in the case of an integrated circuit structure, to achieve low cost of the communication device.